1. Field of the Invention
The present invention relates to a technique for determining a developing bias voltage to be applied to a developing portion when developing an electrostatic latent image.
2. Description of the Related Art
In an image forming apparatus adopting the electro-photographic system, a developing bias voltage is applied to a developing portion so as to develop an electrostatic latent image on an image holding member, thereby forming a toner image. Here, a target bias voltage value of the developing bias voltage for forming a toner image having a target density or a target concentration is not constant at all times, and can vary or fluctuate depending on, for example, any degradation of the toner or the image holding member, etc.
In view of such a situation, there is a conventional image forming apparatus provided with a function for adjusting the bias voltage. This image forming apparatus forms a mark on an intermediate transfer belt, and uses an optical sensor to detect the density or concentration of the mark. Then, the image forming apparatus determines a target bias voltage value based on a predetermined correlation between the density and the bias voltage value of the developing bias voltage. Note that this correlation can be expressed by using a slope in a graph wherein one axis represents the density of the mark and the other axis represents the bias voltage value. Accordingly, the slope is adopted as a parameter indicating the above correlation.
However, due to the degradation of the toner or image holding member, etc., the slope itself varies in some cases. In the conventional image forming apparatus, however, the variation in the above-described slope is not considered, and thus more improvement is desired in the determination of target bias voltage value.